Use a better name for compile unit labels.

Use a better name for compile unit labels.

They mark the start of a compile unit, so name them .Lcu_*. Using
Section->getLabelBeginName() makes it looks like they mark the start of the
section.

While at it, switch to createTempSymbol to avoid collisions with labels
created in inline assembly. Not sure if a "don't crash" test is worth it.

With this getLabelBeginName is dead, delete it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231750 91177308-0d34-0410-b5e6-96231b3b80d8